Hi!
The expander is somehow a mystery for me. The name sounds like
something good and that it could be an improvement for sound.
The gate function is ofcourse clear to me, but what shall I do
with the EXPANDER.

If someone could explain for me how and when I could use it
I would be happy. I play the guitar, so I often have a lot of
acoustic and electric guitars in my songs along with drums and bass.
I´ve been twisting nobs but still don´t get it. Is it me........?
I use the Waves SSL E channel and Digidesign Gate/Exp mostly.

Upward (not as common) increases the level of signals that exceed the threshold. Applications might be a mastering engineer making an overcompressed mix more lively.

Downward (much more common) decreases the level of signals that fall below the threshold...like a gate, except that the expander lets you leave the gate open by a fixed amount. So say you have a snare which you want to gate, and in order to prevent hat spill from causing problems you need to gate quite quickly off the back of the snare....and say that sounds un-natural...you could use the expander to close the signal off by only 6 db or something so that it solves you problem without killing the snare. There are loads of possible uses of course but generally I find that when a gate is too aggressive I try to use the expander/gate idea.

I put it this way - people often compress a signal so the whole thing can be turned up, and the quiet bits are louder. Expanders can kind of turn up the really quiet bits and leave the 'louder' transients as they are - or you could squash that top bit as well. maybe not the most technical way to put it but that's how I got taught and it always made sense.

Try it on a damien rice track or something, loads of ultra-quiet bits that could use it without having to squash down the loudest points and using make up gain.

edit: and of course the other way, as a different style of gate as described above

To me, the beauty about music is that sometimes sounds just speak for themselves, and no matter how hard you try, some of us will never get a satisfying explanation in plain english. Though there's been some great explanations on this thread, none of them can compare to listening to a mix BEFORE and AFTER a properly used expander.

The funny thing is, as confused as we may get, the real explanation of alot of these effects are closer to their name then we give them credit for